Hookworm infection remains a serious disease in rural China, with an estimated 194 million people infected. Current control strategies are limited to costly anthelmintic treatments that require frequent visits, and risk the development of resistant parasite strains. Significant advances in vaccine development are imminent, with testing of a lead vaccine antigen, recombinant Ancylostoma secreted protein-1 (ASP-1) less than 5 years away. In preparation for vaccine implementation, studies investigating the amount of genetic variation in hookworm populations in China are underway. We hypothesize that hookworm populations from geographically separated areas of China will contain significant levels of molecular variation in ASP-1 that could be important in the development of a recombinant hookworm vaccine using this antigen. We will investigate variation in this antigen from different localities across China and compare the results with population genetic parameters inferred using other genetic markers. In most areas of the world, hookworm transmission occurs by the "classic" mode, wherein people are infected by infective stages encountered during activities such as defecation. In China, where farmers continue to use human feces, or night soil, as fertilizer, exposure to infective hookworm larvae also occurs during agricultural activities. The mode of hookworm transmission has important epidemiological and genetic consequences because the small-scale population genetic structure of the parasite should differ under these two modes of transmission. Under the "classic" mode of infection all individuals in a village are infected from a common pool of larvae. Under the "night-soil" mode, families are re-infected by their own pool hookworms. In this case individual members of the same family will harbor worms that are most closely related to each other, causing genetic structure at the level of individual hosts and/or families, which in turn is important for vaccine development and the development and spread of anthelmintic resistance genes. We will use a combination of questionnaires, field observations and molecular genetic approaches to compare the population genetic structure under the two modes of transmission, and to determine the sources of infection under each mode.